9791 Chemistry Example Candidate Responses Booklet WEB

Example Candidate Responses

Cambridge International Level 3Pre-U Certificate inCHEMISTRY (9791)

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Example Candidate Responses

Chemistry (9791)

Cambridge International Level 3 Pre-U Certificate in Chemistry (Principal)

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Cambridge Pre-U Example Candidate Responses

www.cie.org.uk/cambridgepreu 3

Cambridge International Level 3 Pre-U Certificate

Chemistry

9791

Contents Page

Introduction 4

Components at a Glance 5

Paper 2 Part A Written 6

Paper 3 Part B Written 54

Paper 4 Practical 97





Introduction

The main aim of this booklet is to exemplify standards for those teaching Cambridge Pre-U, and to show how different levels of candidates’ performance relate to the subject’s curriculum and assessment objectives.

Cambridge Pre-U is reported in three bands (Distinction, Merit and Pass) each divided into three grades (D1, D2, D3; M1, M2, M3; P1, P2, P3).

In this booklet a range of candidate responses has been chosen to illustrate as far as possible each band (Distinction, Merit and Pass). Each response is accompanied by a brief commentary explaining the strengths and weaknesses of the answers.

For ease of reference the following format for each paper has been adopted:

Mark Scheme

Example Candidate

Response

Examiner Comment

Each question is followed by an extract of the mark scheme used by Examiners. This, in turn, is followed by examples of marked candidate responses, each with an examiner comment on performance. Comments are given to indicate where marks were awarded and how additional marks could have been obtained. In this way, it is possible to understand what candidates have done to gain their marks and what they still have to do to improve their grades.

Teachers are reminded that a full syllabus and other teacher support materials are available on www.cie.org.uk. For past papers and Examiner Reports please contact CIE on [email protected].




Components at a Glance

Component Component Title DurationWeighting

(%)Type of Assessment

Paper 1Part A Multiple

Choice 1 hour 15 Written paper, externally set and marked

Paper 2 Part A Written 2 hours 15 minutes 35 Written paper, externally set and marked

Paper 3 Part B Written 2 hours 15 minutes 35 Written paper, externally set and marked

Paper 4 Practical 2 hours 15 Practical exam, externally set and marked

This booklet contains a selection of example candidate responses and examiner comments for Papers 2, 3 and 4.




Paper 2 Part A Written

Question 1 Mark Scheme







Example Candidate Response – Distinction (D1)







Examiner Comment

The candidate was able to complete almost all of the calculations correctly but failed to read (b) with sufficient care.




Example Candidate Response – Distinction







Examiner Comment

The candidate showed a good understanding of the principles involved in most of the calculations but in (b) failed to convert dm3 into cm3 and made a careless error in (g). Also a lack of appreciation of significant figures was shown in (h).












Examiner Comment

A Distinction level candidate would be expected to achieve full marks on this question.


















Examiner Comment

A Distinction candidate would have probably scored 10/11 marks, with marks for (h) being the most difficult to score.















Examiner Comment

In (a) the candidate was unable to draw the repeat unit of the polymer. The reduction reaction in (c) (iii) was targeted at the most able candidates. A scaling factor of 10 was missed in (c) (vii).




Example Candidate Response – Merit







Examiner Comment

The candidate scored on most of the lower level questions although they failed to attempt (c) (iv) which tested basic knowledge. The overall standard of the answer was improved by scoring in (c) (ii) and for the oxidation half-equation in (c) (iii).








Example Candidate Response – Pass




Examiner Comment

The candidate was able to use their knowledge of atomic structure and s, p, d, f orbitals and extend it to a new element containing a g subshell scoring the marks for (a), (b) and (e). However, the candidate was unable to apply the Aufbau principle to this new element which would have raised their level of achievement.















Examiner Comment

The candidate showed some knowledge and understanding of the chemistry of halogenoalknes and alcohols in recognising the types of reactions 1 and 5, identifying the products of reactions 1 and 2, and recalling a reagent to oxidise product A.









Examiner Comment

Although the overall performance on this question was excellent the candidate made a serious error in the basic chemical knowledge required for (b) (i).








Examiner Comment

The candidate showed an understanding of electronegativity values and correctly predicted the dipole in an N–I bond. In (b) (i), (b) (ii) and (c) the candidate was able to write the balanced equations using the information given but was not able to write the more difficult equation in (c) (ii). Part (b) (iii) was targeted at the lower grades but the candidate only scored 2/3 marks. The candidate showed an understanding of bond lengths/strengths in a covalent compound but was unable to use the information in (c) (iii) to write an ionic formula, the more accessible and widely scored mark.



























Examiner Comment

This good Distinction candidate has only dropped 2 marks. Writing ‘butaenoic’ in part (b) was a common error even at Distinction level. Part (h) (iv) was targeted at the most able candidates and gaining 3 out of the 4 marks here was a good achievement.




















Examiner Comment

The candidate performed well in part (h) (iv) suggesting how a relevant but unfamiliar calculation could be carried out but was unable to do the more accessible calculation in part (h) (ii). An incorrect answer was given to part (d) which would not have been expected at this level.




Paper 3 Part B Written


















Examiner Comment

The salvage mark for an overall ionic equation was earned in (a) with full marks in (b) and (c). Part (d) was generally answered well but confusion between rate and equilibrium lost marks in part (iv).

As with many candidates, part (e) (i) earned full marks but the buffer calculation in (e) (ii) proved more challenging, although 1 mark was earned here for the initial pH calculation.














Examiner Comment

This question is from a script earning a solid Distinction overall.

As with many candidates part (a) proved tougher than expected and this became a high level discriminator. A mark was allowed here for the overall equation but two separate steps were needed.

Part (b) was correct and the only mark lost in part (c) was for the incorrect ratio. This was careless given the equation in (a) and ecf was allowed from there on.

In part (d) surprisingly few candidates recognised the reason for leaving [H2O] out of the Kw expression and the suggestion here (that its concentration is 1) was often seen. Correct calculations in parts (iii) and (iv) gained full marks.

Most candidates were correctly able to calculate the pH in part (e) (i) but part (e) (ii) was another high level discriminator with many candidates failing to recognise the context of a buffer calculation.

The clarity of exposition in part (d) (iv) gives evidence of solving chemical problems and rationalising unfamiliar material while the clear calculation in part (e) (i) also gives evidence of good problem solving.


















Examiner Comment

Overall this script fell in the low Merit band.

Many candidates failed to remember the crucial distinction between elimination and nucleophilic substitution of halogenoalkanes being in the choice of solvent and temperature.

The more familiar definitions were remembered in part (b) but, again as with many, the less familiar diastereoisomers caused problems and was confused with enantiomers. It was rare to see such clear 3-D diagrams in part (iv) but 2 marks were earned in this case.

Clear use of curly arrows and unambiguous representation of inversion were the keys in part (c) (i) but there was no inversion shown here and, in part (ii) the clue in the stem failed to stimulate the correct response of a full systematic name with appropriate prefixes.















Examiner Comment

A Distinction level response; albeit one that started poorly. Clear references to states of matter and molar quantities were needed with the general advice being to be as specific as possible with references to given data.

A slip in part (b) (i) was the only other source of lost marks in this response with the calculations clear and a recall of the concept of the activation barrier in part (d).











Examiner Comment

Overall an outstanding Distinction level script, which therefore shows what a high level discriminator question 3 (a) proved to be. This candidate only earned 1/4 here as, although the end result was mentioned in part (a) (i), for each reaction there was no mention of the change involved.

Again, the key to success here is a specific, detailed description of the state changes that occur.

A slip in transposing data in part (c) (ii) was the only other source of a lost mark in this question but does illustrate the need for care at all levels.











Examiner Comment

In part (a) there was no evidence of any recall of the key principles governing the sign and magnitude of entropy and the candidate seemed to be confused between entropy and enthalpy.

The calculations in part (b) were done well but, in part (c) (i) the candidate failed to convert ΔrG to Joules and, in (c) (ii) defining ΔrG as zero and proceeding from there was not seen.

The candidate also failed to recognise the concept of the activation energy barrier in part (d).

Consistent use of appropriate significant figures throughout part (b) provide evidence of a concern for accuracy and precision.












Examiner Comment

This was a good Distinction level script with this response exemplifying the overall standard.

In part (a) the development of the empirical formula to the molecular formula using the relative molecular mass was clearly shown and the only mark lost was for a slip in the structures – possibly the ‘squashed up’ molecular formula hindered back checking.

Part (b) again highlighted the need for specific recall of some example ions whilst part (c) was again only marred by an explanation couched in terms of emission.

The quality of responses to part (a) and part (c) suggest an understanding of chemical reactions, structure and the use of a model for describing colour of complexes.








Examiner Comment

Overall this script was at the level of a high Merit and this question in itself was around the Distinction boundary.

The empirical formula was calculated correctly but the candidate failed to justify a molecular formula or to draw the isomers, which may have been due to not reading the question carefully.

An incorrect charge for CoCl42- cost a mark in part (b) whilst a generally good account of colour in part (c)

was only marred by a reference to emission of coloured light as the promoted electron ‘dropped’, when what is required is a reference to absorption leaving the complementary colour.








Examiner Comment

Overall a Merit level script with this response on a par with the overall performance.

Again there was no progression from the empirical formula to the molecular formula shown in part (a) and unfortunately this candidate failed to recognise that N2H6 represented two NH3 groups so was unable to make a sensible suggestion for the structure.

Part (b) illustrates the need for candidates to remember some example material for reference in questions. The concept of split energy levels was remembered in part (c) but then the origin of colour was explained in terms of emission rather than absorption.








Examiner Comment

Overall this script was just below the Merit borderline and this response exemplified the overall performance.

Specific recall of some platinum complex ion chemistry is expected (syllabus B2.4 (d)) but the candidate represented the complex as tetrahedral. The presence of only two different groups should also have served as a clue that a representation of optical isomerism would be inappropriate here. The other mark lost was for failing to show that the molecular formula was the same as the empirical formula, by reference to the relative molecular mass.

Omission of charges in part (b) was costly and part (c) required the specific recall of a principle that needs committing to memory.


















Examiner Comment

Parts (a) – (d) were generally well answered as was the case for most candidates. A Distinction level candidate would have been able to identify all three functional group levels in part (c) and both bond angles in part (b).

Part (e) was definitely a high level discriminator. Only 2 marks were gained for the structures and they came from ecf from incorrect original structures as the true nature of the reduction process was not recognised. Failing to distinguish between a peptide bond and an amide cost a mark early on in the description.

References to NMR spectra need to be clear and specific and the most often seen problem with references to 13C spectra was, as here, with candidates who seemed to be referring to bonds as being responsible for the spectrum rather than to carbon atoms/nuclei. This could have been due to confusion with UV spectroscopy or simply poorly phrased answers.














Examiner Comment

As with many candidates a reasonable attempt was made at parts (a) – (d) although the clue in the stem of referring to addition polymerisation was overlooked as this candidate attempted some form of condensation polymer in part (d) (ii).

Correct identification of R in part (e) should have led to a correct structure of Q but a mistake was made with regard to the chain length.

The explanations incorrectly referred to an acyl group as a carbonyl group and again lacked specificity when referring to proton NMR although the 13C NMR was well explained.




Paper 4 Practical

Mark Scheme


















Examiner Comment

(a) The candidate scored all 8 marks in the opening part although it was not necessary to record the mass of residue and the mass of water lost following each period of heating as was done here.

(b) Again the candidate gained full marks here. Although they do not state the relative formula mass of MgSO4, they have clearly shown how the value is obtained and gain the mark.

(c) The correct uncertainty was given in part (i) but the candidate did not double this value in part (ii) as it was not appreciated that two masses were needed to work out the mass of water lost, these being the combined mass of the crucible and FA 1, along with the combined mass of the crucible and the residue.

(d) This answer was too vague to merit any credit. The candidate goes someway to suggesting the use of a higher precision balance but this does not merit credit.











Examiner Comment

(a) The candidate scored all 8 marks on the opening part. All the results were clearly displayed in a single table including the mass of residue and that of water lost. The data was noted to an appropriate level of precision. The headings were clear and units given. In addition, the final masses lie sufficiently close together and the results are in excellent agreement with those of the Supervisor.

(b) The second mark for showing the mass of water lost divided by the relative formula mass of water given despite the slip of quoting the answer as 0.439 rather than 0.0439. The ratio was allowed despite the incorrect rounding of the answer which uses the two calculator values for the amount of water and residue rather than the values noted in the working.

(c) As the candidate was using a balance that read to two decimal places, the answer should have been given as ±0.005 g. Another mark was lost by not doubling the value from part (i) in the following calculation. The candidate did not appreciate that two masses are required to work out the mass of water lost.

(d) An excellent suggestion for an improvement and furthermore, the candidate gave a clear explanation of why the suggested improvement would lead to a more accurate value of x.











Examiner Comment

(a) The candidate scored both recording marks by recording the mass to the appropriate number of decimal places and by including the correct units in the column headings. The column recording the time was superfluous. Two further marks were awarded for the candidate recording final masses after heating that fell within 0.05 g of one another. The final mark achieved for this section came from the level of agreement between the candidate’s ratio of mass of water lost to mass of residue with that of the Supervisor’s value. The agreement was not sufficiently close to award the second available mark. The candidate did not record all their results in a single table nor did they calculate the mass of the residue.

(b) The candidate used their values from part (a) to correctly calculate an integer value of x. They have also clearly shown their working and so gained all 4 marks.

(c) (i) The candidate may have interpreted this question as asking for the uncertainty in a single recording of mass or the uncertainty in the value of one of the masses. Acceptable answers for this candidate, who used a balance that read to three decimal places, would be either ±0.0005 g or ±0.001 g.

(ii) The candidate approached the question incorrectly by working out the maximum and minimum values of x that would arise from their quoted uncertainty in the mass of water lost.

(d) The candidate’s final comment was correct in that a more accurate value of x could be obtained by increasing the mass of FA 1. However, an incorrect answer had already been given in suggesting that accuracy could be improved by heating for shorter periods of time.











Examiner Comment

(a) This candidate did not record the mass of water lost as instructed and so did not gain the first mark. This also meant that they were unable to gain the sixth marking point for correctly calculating the mass of water lost and the mass of residue. The marks for recording all mass readings to an appropriate level of precision and with the correct units were awarded, as were the marks for heating to near constant mass. This candidate also scored both marks for the quality of their data compared to that of the Supervisor.

(b) The candidate’s method was correct but at one stage the candidate rounded the number of moles of magnesium sulfate to just one significant figure which leads to an incorrect ratio using the data from part (a).

(c) The uncertainty was correctly given in part (i) but the candidate did not appreciate that in order to work out the mass of water lost, two measurements of mass are required and so the uncertainty must be doubled before being used in the calculation in part (ii).

(d) The suggestion to repeat the experiment was a response given by a number of candidates but did not merit any credit as it does not necessarily lead to a more accurate value of x.












Examiner Comment

(a) Overall this is a very good answer with clear recordings of all observations and sound use of the appropriate technical terms. The only mark not to be awarded was the one for noting that the white precipitate formed by mixing FA 2 with FA 6 was soluble in an excess of FA 2. It was apparent from this candidate’s answer that they added too much FA 2 in the first place so their initial white precipitate dissolved almost at once.

(b) All the ions were correctly assigned.

(c) As occurred on many of the scripts only the first mark could be awarded here. The second mark in this part relied on candidates realising that both nitrate and nitrite salts evolve ammonia on warming with aluminium and sodium hydroxide solution. To be certain that a nitrate was present it would be necessary to note that no pale brown gas was liberated when the sample was treated with dilute acid.








Examiner Comment

(a) The candidate observed the correct initial products when combining FA 2 with the other unknown solutions but does not go on to note that the white precipitate formed on mixing FA 2 with FA 6 is soluble in an excess of FA 2. No precipitate results from the mixing of FA 3 and FA 4 as recorded by the candidate and only shades of yellow were acceptable as descriptions for the colour of barium chromate, formed by mixing FA 3 and FA 5. Despite not being awarded the mark for the colour of barium chromate, the use of bright to describe the colour of lead chromate was sufficient to merit the mark for the comparison of the two shades of yellow. Overall the candidate uses slightly confusing terms to describe the observations. For example, it is not necessary to describe a precipitate as being opaque although this did not mean the loss of any marks. In the final part, however, with the mixing of FA 5 and FA 6, the candidate stated that a white saturate was produced which was deemed insufficient to convey the sense of a precipitate.

(b) All ions were correctly assigned.




(c) The second mark in this part relied on candidates realising that both nitrate and nitrite salts evolve ammonia on warming with aluminium and sodium hydroxide solution. To be certain that a nitrate was present it would be necessary to note that no pale brown gas was liberated when the sample was treated with dilute acid.





Examiner Comment

(a) All the marks were awarded for the observations of FA 2 with each of the other unknowns, apart from the solubility of lead hydroxide in an excess of FA 2. For the observation on combining FA 3 and FA 4, although the colour was given correctly, the description of the solution as an emulsion meant that the mark was not awarded. The candidate also described the colour of barium chromate as lime green which is incorrect. The mark for noting the colour of lead chromate was awarded as the candidate has recorded it as being different to the colour of barium chromate.

(b) The candidate incorrectly assigned the barium ion to FA 2 despite noting correctly that FA 2 did not form a precipitate with FA 3 which was correctly identified as containing the chromate ion. The candidate also incorrectly assigned the sulfate ion to FA 5 despite having correctly assigned FA 4 as containing H+ which would not have given a white precipitate on mixing with sulfate. The source of these errors lies in the observation of a white precipitate with FA 2 and FA 5 which can occur if the solutions are not pure.

(c) The candidate failed to pick up either mark here. They forgot to note that aluminium as well as sodium hydroxide solution must be added before warming if ammonia gas is to be liberated. The second and much more challenging mark was for noting the need to distinguish between nitrate and nitrite which the candidate does not do.








Examiner Comment

(a) Two marks were not awarded in part (a). The candidate did not test the solubility of the precipitate of lead hydroxide, formed by mixing FA 2 and FA 6, in an excess of FA 2 and so did not observe that it was soluble. On mixing FA 5 and FA 6 the candidate records a milky solution rather than a white precipitate or white solid.

(b) The candidate assigned the hydroxide ion to be present in FA 5 despite noting correctly that FA 4 and FA 5 when mixed give a white precipitate. FA 2 was stated as containing sulfate ions. This error arose from not having noted the solubility of the lead hydroxide in an excess of hydroxide solution which would rule out the white solid as being lead sulfate.

(c) The candidate correctly describes a positive test for nitrates or nitrites but then does not go on to say how adding dilute acid and looking for the presence of brown fumes can distinguish between them.












Examiner Comment

(a) The candidate set out the results in a very clear format. It was not necessary to note the colours both before and after warming each solution with acidified dichromate but so doing clearly indicates a student who is observing closely and working carefully through the series of tests. All observations were correct so the candidate gained full marks for this part.

(b) The identities were given correctly and there was a sound explanation of how each assignment matches the observations made in the opening part.








Examiner Comment

(a) The candidate constructed a clear table for the results and so gained the first mark. Although the observation was correct for FA 7 and Tollen’s reagent, it appears that there was no reaction between FA 7 and the acidified dichromate solution. Here the candidate’s background knowledge let them down as clearly none of the possible compounds could give such a result. The other observations with acidified dichromate were correct although no detail was given about whether the change happened before or after warming. The use of N/A to indicate no reaction is not recommended when describing what happens on mixing each reagent with Tollen’s solution.

(b) The assignments were correct but credit can only be given when they are supported by the correct observations. Hence, it is insufficient to say that FA 8 is an alcohol simply because it reacted with the acidified dichromate as this would also be true for the aldehyde. The assignment of FA 7 is not supported by the observation of no reaction with acidified dichromate in part (a). Consequently, only the mark for assigning FA 9 to the ketone was awarded.




Example Candidate Response – Unclassified




Examiner Comment

(a) The first mark was awarded for presenting the results in a single table. The candidate did not gain any of the 3 remaining marks as one of the pair of observations for each unknown was incorrect. The candidate’s background knowledge was clearly unsound as the observations for FA 9 could not be true for any of the possible unknowns.

(b) No marks could be awarded here as in each case the assignment does not match the observations made in the opening section.

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9791 Chemistry Example Candidate Responses Booklet WEB